In recent years, display devices comprising thin-film transistors (TFTs) as semiconductor devices have been put into practical use. Examples of such display devices include liquid crystal displays and organic electroluminescent display devices. Generally, a TFT comprises a semiconductor layer of amorphous silicon, polysilicon or the like. More recently, as for the TFTs, a structure comprising an oxide semiconductor layer, a typical example of which is indium oxide gallium zinc (IGZO), has been intensively researched.
For example, with respect to liquid crystal displays, there is a demand for a technology to perform intermittent driving at low frequency to achieve low power consumption in still image display. As a switching element for pixels of liquid crystal displays, a TFT comprising a semiconductor layer of polysilicon is known. In this TFT, the pixel voltage applied to the liquid crystal varies because of the leakage current produced when the video signal is in off state. For this reason, if the pixels are intermittently driven at low frequency, a problem may arise wherein flicker becomes noticeable.
A conventional measure for the above-described problem is a method of reducing variation in pixel voltage by providing a sufficiently large retention capacitance in parallel with a light-shielding electrode and liquid crystal. However, as pixels continue to become smaller, it is becoming difficult to provide a large retention capacitance.